Just had my new (to me) 2010 6 speed MT in for its first oil
change at 5500 miles. The mechanic suggested that at around 10k miles it
might be a good idea to change the diff fluid because there could be metal
shavings in it since the car's new.

I'm not mechanical enough to know whether this is a good idea or a waste of
money. What do you guys think, and why?

Some do change early and it won't be harmful, however the mazda service schedule lists at 48 months or 60,000
miles. I did mine early at around 12,000 miles as I wanted to upgrade the
brand used. I personally won't make/pay for a service visit just to do this
however if it is already going to be in then I might have it done. More info
from mazda below:

I changed the rear diff lube around 10,000 miles in my 06. I
wanted to change to synthetic, and this time frame worked out well. I have
the T/F limited slip diff.

The fluid was really black. "I have heard" some of the blackness
came from the ring and pinion gears seating, and some from the clutch packs
seating into each other. As black as it was, I am glad I changed it if for
only that reason.

At 20,000 miles I put the Motorcraft XT-M5-QS in my
6 spd tranny, and thought while it was up on jack
stands I might as well drain out the rear diff and refill it with the left
over differential lube I had from before. The fluid that came out was only
darkened a little.

My one uncle used to be in charge of a fleet of vehicles for the state, he
always said that "oil and grease is cheaper than parts and labor".
I tend to follow that line of thinking too.

If it's a standard diff on that 06 it probably is not
critical, because it's not so precise and
complicated as a limited slip version. The drain plugs on the diff and tranny
have a magnet that will catch and hold filings anyway. Surprised he did not
suggest the tranny ... unless he was nervous about it. Changing that fluid
would be better. Search here for info on "Motorcraft
Full Synthetic" for loads of info about tranny fluid for you. Even if
you are not having troubles, it's nice to put some quality stuff in there for
the long run.

Before you do the next oil change, post back with a price that they ask
change the diff and tranny fluids. Folks will let you know if it is
reasonable. It's an incredibly easy job to do yourself
with a $10 hand pump to put the new fluids in. But it does involve getting
the car up in the air, crawling under, etc. Depending where you are in CA, I
bet you could find a member nearby to do it with you, and give you a guided tour
of the bottom of your car at the same time. Well worth the effort! In fact,
consider doing that for your next oil change, and installing a Fumoto valve
at the same time. It will help prevent future greasemonkeys
from screwing up your oil drain plug.

Welcome to miatadom!

__________________

When something goes
wrong, I'm the first to admit it and the last one to know.
~Paul Simon

.... I'm not mechanical enough to know whether this is a
good idea or a waste of money. What do you guys think, and why? ...

I changed mine at about 3k miles and glad I did. The old stuff
came out black and sludgy, and there was metal present. Not saying
necessarily that this is a problem, but I'm sure the baby appreciated the
clean new stuff.

Used one quart (liter?) of Mobil1 synthetic GL5 gear oil. Cost ~$10. Took
about 1/2 hour to change. It came in a sorta
squeeze bottle with spout, so no funnel or pump was necessary.

Changed it again at ~20k miles. Surely overkill, but babies are meant to be
babied. And it's sorta satisfying to do these
simple jobs yourself.

A much more useful alternative to the F. valve, is a
"suck it out the dipstick" device. It's on my short list of must
have tools. Right up there with the clamp-on DC meter and the digital vernier calipers.

I'd make sure you work the clutch packs a little before you
dump the fluid if you have a LSD. Want to get all the break in wear over with to old oil so the new oil
will stay clean longer.

If you have no LSD, dump it when it's convenient.

You'll like the synthetic oil on cold mornings. I know my drivetrain feels
like glue first thing in the morning when it's in the 20F's out. Why did
Mazda use conventional oil in the gearbox and diff???

Differential oil change on a new rear…..You should change the
fluid at around 600 to 1,000 miles.
You say why??????

You are breaking in a new gear box the first few hundred miles are going to
make the most friction and wear in the shortest period of time than the full
life of the rear end.
This is true of the open standard rear or the LSD equipped rear. The LSD rear
has a little bit more friction going on than the standard open but both types
of 3rd member share all other same components.

The NC and NB Miata rear takes just shy of a quart/liter to fill….over
filling is bad….Why????? Because the oil can foam from lack of ventilation.
Filling the case is easy….fill to the oil starts to come back out the fill
hole. Let the oil flow back out on a level surface till it slows to a
trickle/drip then replace the fill plug. The rear has a proper level doing it
this way and you have no second guessing if it is right.

What should I use for oil????? A good 75/110 or 75/140 standard or synthetic
oil as long as it is a GL-5.
Stay away from Royal Purple gear lube use wall mart brand before this over priced substandard stuff.
But my book says 75/90?????? The 75/90 of old is the new 75/110 the 75/110
has taken the place of the 75/90 in lubricating properties.

What brand of oil should I use? Amsoil brand is the best and the best priced
for the top of the line lubricants. Mobil 1, Red Line, Valvoline are also
very good choices.

Change the fluid again in 5000 miles doing this will get the rest of the
break in debris out. Refill and change ones a year or 2. The total cost of
this doing it your self is about $45 on the high
side and $30 on the low side. It’s cheap insurance to assure a proper long
lasting gear box.

Be sure to inspect and clean the magnetic drain plug. If you see large metal
pieces or heavy metallic in the oil this is cause for inspection under
warranty. There is always going to be some metallic on the drain plug and in
the old oil this is normal.

Do a little research on all I tell you, you will understand I’m not giving
opinion just fact.

Point of info for new readers on the forum: S2KPuddyDad is a
specialist in rear ends, and we're talking about the metal kind in his
avatar, not the other kind. So his post here is the authoritative final word.
Ignore everything hacks like me said before.

__________________

When something goes
wrong, I'm the first to admit it and the last one to know.
~Paul Simon

I don't understand the logic of using 75W-140 instead of
75W-90. Everyone seems to be moving to lighter oils that reduce drag (5W-20
instead of 5W-30 for example). Why the reversal of the trend?

There is nothing trendy about gear box lubricant. The design
of the hypoid differential gear box is old school and has not been improved
on like the engine and transmissions have been over the years.

The Mazda gear box differential calls for a SAE 90W from the factory using a
75/90w may not give adequate protection. Your safe bet is a 75/110 GL-5
hypoid oil.

In 2006 the oil standards changed Please read,

“The need for redefined classifications centered on the wide variation in
kinematic viscosity possible for SAE 90 and SAE 140 grade lubricant. In the
prior version of SAE J306, these ranges were so broad that lubricant
viscosity could vary greatly and still technically be in grade. Two new
viscosity grades were introduced to cover the upper end of the specifications,
SAE 110 and SAE 190. The more narrowly defined classification provides OEMs
with greater flexibility in specifying a viscosity grade to ensure an optimal
balance of fuel economy and durability for their equipment.

In addition, under the old designations, an axle could be serviced with a
lubricant having a viscosity significantly lower or higher than the lubricant
with which the axle was validated. To prevent the axle from being filled with
a lubricant with too low a viscosity, an OEM was forced to specify a higher
viscosity grade than actually desired.

For example, under the old classifications, if the optimum 100°C viscosity
for an axle was 19.5 cSt, the OEM would normally
specify an SAE 90 lubricant. However, the actual viscosity of this lubricant
could be as low as 13.5 cSt, which might be too low
to provide the required durability. To prevent this problem, the OEM would
recommend an SAE 140 grade lubricant, ensuring that viscosity would never
fall below 19.5 cSt. Unfortunately, that also means
viscosity could be as high as 41.0 cSt, possibly
resulting in poor fuel economy or shift problems. Under the new limits, the
OEM would recommend SAE 110 grade lubricant, which meets the 19.5 cSt requirements and ensures that the axle is serviced
with a lubricant having a viscosity no higher than 31 cSt.

A Question of Balance

OEMs are challenged today to meet the often
conflicting demands for improved fuel economy coupled with improved axle
durability. Government regulations are dictating better fuel economy, while
vehicle owners are demanding increased performance. Engine horsepower has
increased by 34% over the last decade, but axle gear sizes have remained
constant, sump capacities have been reduced, and drain intervals have been
extended. These demanding conditions can be met only by axle lubricants that
provide enhanced durability protection.

Axle efficiency can be improved by using lower viscosity fluids (such as SAE
75W-90), which minimize frictional churning losses. However, light trucks and
SUVs are often used in conditions that expose axles to heavy loads or high
operating temperatures. These conditions require superior axle protection,
often provided by choosing a heavier viscosity fluid (SAE 75W-140, for
example). Commercial vehicle applications may have even more stringent
durability requirements. However, providing high durability and long vehicle
life often reduces fuel efficiency.

The new viscosity classifications provide more flexibility for OEMs to select
the optimum viscosity grade for an application. For example, the SAE 110
grade should be useful in an SUV or light duty truck where improved fuel
economy is important but high torque or durability at high speed operation is
essential. This new classification should aid OEMs in specifying a fluid
viscosity more tightly to ensure that the lubricant has the intended physical
properties for a given application.

The SAE 190 grade may allow better efficiency and enable multigrade
performance in severe duty applications where high film strengths are needed.
These end uses may include applications such as construction equipment
operated in extreme temperature environments. The end result is a better
match of fluid performance to axle needs.

The Mazda gear box differential calls for a SAE 90W from the
factory using a 75/90w may not give adequate protection.

I fail to see how a 75W-90 oil
doesn't meet the viscosity requirements of a SAE 90 oil. The 90 is the
important number in 75W-90, it means it conforms to SAE 90 specs at operating
temperature. The 75W only means it's thinner than SAE 90 when cold, like yesterday
when it's 18F outside here. That SAE 90 gear oil felt like glue.

Anyway, back on topic. Since it says the manufacturers are specing thicker oils than they need so their diffs always
have oil thick enough, it seems reasonable to assume that any SAE 90 should
fit the bill. However, the real question here is what is the the viscosity of the OEM oil because then it should be
fairly easy to find a multi-grade synthetic to match.

I fail to see how a 75W-90 oil
doesn't meet the viscosity requirements of a SAE 90 oil. The 90 is the
important number in 75W-90, it means it conforms to SAE 90 specs at
operating temperature. The 75W only means it's thinner than SAE 90 when
cold, like yesterday when it's 18F outside here. That SAE 90 gear oil felt
like glue.

Anyway, back on topic. Since it says the manufacturers are specing thicker oils than they need so their diffs
always have oil thick enough, it seems reasonable to assume that any SAE 90
should fit the bill. However, the real question here is what is the the viscosity of the OEM oil because then it should be
fairly easy to find a multi-grade synthetic to match.

It is a long read but the answer is there. Some brands of
75/90 will fall below safe standards for our rear requirements. If you wish
to use 75/90 go to your brand of choice web site and find out if it meets the
standards many do not.

Or even more easy…..USE 75/110 it’s a no brainer

Just a little FYI to you track junkies use 75/140 or an SAE 190 the stock LSD
will perform much better

It is a long read but the answer is there. Some brands of
75/90 will fall below safe standards for our rear requirements. If you wish
to use 75/90 go to your brand of choice web site and find out if it meets
the standards many do not.

My question is what are the "safe" standards? What cSt at 100C? Will the 15.2 cSt of M1 75W-90 meet the specs or does the oil need to
be thicker?

EDIT: Basically, I may not know everything about rears, but I know a lot
about oil. I'm open to using non-factory spec oil, but not without damn good
reason. The gear oil standard change has been in effect as long as the NC has
been around, so I don't really buy the "110 is
the new 90" argument.

My question is what are the "safe" standards? What
cSt at 100C? Will
the 15.2 cSt of M1 75W-90 meet the specs or does
the oil need to be thicker?

EDIT: Basically, I may not know everything about rears, but I know a lot
about oil. I'm open to using non-factory spec oil, but not without damn
good reason. The gear oil standard change has been in effect as long as the
NC has been around, so I don't really buy the "110 is
the new 90" argument.

Since our rear requires a SAE 90W you need a cst of 19.5
This is why the change and why the old 75/90 is the new 75/110
You need to use 75/110

Also note your owners manual states no 75/90
available through Mazda for your differential
SAE 90W or 80/90W as a second
2 main reasons for diff failure is driver error and inadequate lubrication

See, this is the disconnect here.
Because if I read the specs right SAE 90 should be a minimum of 13.5, not
19.5. So how do you know they meant 110 when they said 90? Maybe they meant
90? The MX-5 isn't exactly a torque monster.

See, this is the disconnect here.
Because if I read the specs right SAE 90 should be a minimum of 13.5, not
19.5. So how do you know they meant 110 when they said 90? Maybe they meant
90? The MX-5 isn't exactly a torque monster.

Most 75/ 90w will fall below 13.5 this number is not a safe minimum.
Standards are 19.5 for your Miata differential.
The Miata diff case only holds about 900ML of fluid not much for a lot of
heat and shear going on inside our differential.
If you need more reassurance please do more research on your computer or just
use what you think is best....Good luck

See, this is the disconnect here.
Because if I read the specs right SAE 90 should be a minimum of 13.5, not
19.5. So how do you know they meant 110 when they said 90? Maybe they meant
90? The MX-5 isn't exactly a torque monster.

If you were to ask over in the lubrication section you might
get more answers. Whether or not they'll agree with what's been recently
said, I have no idea, but it will make an interesting read.

Since 2006 75/90W in general (check your brand of use for there tech info) is no longer meeting the minimum safe
operating standards for the differential. This is not my doing or is it
because I say so.....IT IS FACT BY YOUR MANUFACTURER AND BY THE OIL INDUSTRY
AT LARGE.
You can continue to use 75/90W and find no notable effect or issue but you
take that chance.

Be an informed consumer check the brand of oil that you use web site. Check
the numbers (all of them that concern your use) then at that point you will
know if the brand and type/blend of oil in question is the right oil for your
needs.

Please adjust your thinking.......This is what the oil people want you to do!
75/110W is now the Minimum safe oil to use if your gear box required 75/90W

You guys do as you please you have been informed and given good information
not opinion

Most 75/ 90w will fall below 13.5 this number is not a safe
minimum. Standards are 19.5 for your Miata differential.
The Miata diff case only holds about 900ML of fluid not much for a lot of
heat and shear going on inside our differential.
If you need more reassurance please do more research on your computer or
just use what you think is best....Good luck

See, I use synthetic oil so I'm not worried about the oil not
being the viscosity that it's supposed to be. Mobil 1 is pretty on the ball
with such things.

Now you say that 19.5 cSt is the spec, which is
fine by me, but how do you know this? Where did 19.5 come from?

See, I use synthetic oil so I'm not worried about the oil
not being the viscosity that it's supposed to be. Mobil 1 is pretty on the
ball with such things.

Now you say that 19.5 cSt is the spec, which is
fine by me, but how do you know this? Where did 19.5 come from?

synthetic or standard oil makes
no difference the numbers are the numbers.
Please check the manufactures specs and minimums of the blend you use.

The number is the standard for this type of gear box.

I can't keep posting it is getting redundant. At this point please others
chime in with fact not opinion. All that are in question about the brand oli and blend they use check your manufactures spects and minimums or if you have no time for this
debate just use 75/110W again it's a no brainer

You keep asserting the 19.5 number, but don't tell us where to check that.

I don't mean to be short or be none informative about a
question. I called Honda a few years back and asked. You can call Mazda and
do the same. The Mazda, Honda, KIA along with others use the Mazda 3rd member
design.

The oil manufacturing industry it putting the information out to all auto manufactures the changes in oil ratings.
Everything is changing Toyota now uses a straight
weight engine oil in all new autos and trucks being made now.

So basically you don't know what Mazda recommends for the
NC's diff. That 19.5 business was just made up.

Wow....the Mazda rear is the same as the Honda the number is Honda and will
translate to Mazda as it is the same.

This is just not our rear it is the type of gear box in general.

I don't get it, it is explained in the TSB I posted.
I don't make this crap up I'm not educated at this level. You take the
information your given and you pass it along. Don't shoot the messenger.....
The TSB states that MOST 75/90 oils will fall below Industry standards for
minimum protection. For me and my 25k investment I will take the information
and use the 75/110 as the oil companies tell me.

It appears that the OMs and WSMs for NCs of all years
recommend 75W-90 or 80W-90 for the diff.

Is Mazda wrong in this, or out of date? Surely if Mazda thought 75W-110 was
required they would have said so.

Is Mobil wrong in saying that their Synthetic 75W-90 is suitable for
differentials?

This is puzzling.

I call it a TSB because it reads as such....

Our owners book says SAE 90w or 80/90w (75/90* not available from Mazda)

The oil companies changed ratings to better narrow the range of use and to
bring it all under a global standard as I understand it.
The 75/110 is more in line and in the range of standard use now.
75/140 for trucks SUV and towing and SAE190 SAE250 for sever use.

Take note to your auto parts stores gear oil selections….Before it was 90w or
75/90 now you have up to a half dozed blends to chose from.

So basically you don't know what Mazda recommends for the
NC's diff. That 19.5 business was just made up.

Quote:

Originally Posted by wooo

It appears that the OMs and WSMs for NCs of all years
recommend 75W-90 or 80W-90 for the diff. Is Mazda wrong in this, or out of
date? Surely if Mazda thought 75W-110 was required they would have said so.

Remember that this is the same Mazda that recommended GL4 OR GL5
for the tranny (do they still?), even though many of us have discovered that
GL5 is problematic for the syncros and the GL4 Motorcraft cleans up a GL5 problem in time.

__________________

When something goes
wrong, I'm the first to admit it and the last one to know.
~Paul Simon

Remember that this is the same Mazda that recommended GL4 OR GL5
for the tranny (do they still?), even though many of us have discovered
that GL5 is problematic for the syncros and the
GL4 Motorcraft cleans up a GL5 problem in time.

At
this point I don't have an opinion on the matter. I am just trying to gather
facts on which to base an opinion.

I'm not getting an answer on why Mazda is still recommending 75W-90, well
after the GL5 spec was revised and new categories created.

S2K seems to be saying that an oil with 15 cSt is too thin to be used in diffs, yet he then points
us to the Mobil 75W-90 spec sheet.

I posted Mobil 1 specs for you to see not to recommend. I also
posted Amsoil to compare with some explanation of the range of theses blends.

I have not evaded your question(s) I think I answered them to the best of my
ability and recollection. I never talked or Emailed the engineers at Mazda I
did talk to customer service at Honda motor Co USA some years ago and was
given a range of 19 to 32 cst as a safe range for
general use of the S2000 Differential. I was told to use SAE90W as per my
S2000 owners manual. Maybe
the person I talked to was wrong I don’t know???

Honda dealers never sold and Honda motor Co never made a
differential oil for the S2000. There are countless horror stories of
the dealers using Honda CRV transaxle fluid in S2000 rears. The owner did not
get down the road far before the rear would fail.
My S2000 manual does not give any other oil choice but GL-5 SAE90W

The oil companies are putting this out there for us to know that you have
better choices. They are telling us that 75/110 will protect our investment
better with out compromise.

At this point I don't have an opinion on the matter. I am
just trying to gather facts on which to base an opinion.

I'm not getting an answer on why Mazda is still recommending 75W-90, well
after the GL5 spec was revised and new categories created.

He may very well be correct in suggesting a heavier
oil for the diff but he's doing a poor job of convincing me of that.

+1

I was thoroughly confused by this thread, and a bit concerned, because I just
replaced all the differential and transmission oils
in my two Subarus with 75w90 because that's what's
specified.
(Don't even ask why a guy called Bimmer on a Miata
site has two Subarus.)

So, I went to BITOG and found this:

Before 2001 or so these are the grade ranges of 90 and 140
90: 13.5cSt to <24cSt at 100C (212F)
140: 24cSt to <41cSt at 100C

Now:
90: 13.5cSt to <18.5cSt at 100C
110: 18.5cSt to <24cSt at 100C
140: 24cSt to <32.5cSt at 100C
190: 32.5cSt to <41cSt at 100C

FWIW, this is from a thread on BITOG titled "Ford switch 80w-90 to
75w-140 synthetic." Of course, Ford made that switch in 2001 or so...

I still have no idea where Puddy's 19.5cSt number
came from, but the MAXIMUM viscosity for 90-weight oil seems to be 18.5cSt @
100c, so if Honda really spec'ed a 19.5cSt, then
that would indeed be a 110-weight oil.

Otherwise, if since '01 (or better, since the NC appeared in '05) Mazda says
75w90, then they presumably mean 75w90, not 75w110
or anything else.

And I just checked, and the Redline 75w90NS I put in my transmission has a
viscosity of 15.6cSt, and the Redline 75w90 for the differentials is 16.4.
I'll sleep well tonight!

I still have no idea where Puddy's 19.5cSt number
came from... the maximum viscosity for 90-weight oil seems to be 18.5cSt @
100c.

Bimmer

customer service at Honda motor Co USA some years
ago and was given a range of 19 to 32 cst as a safe
range for general use of the S2000 Differential. I was told to use SAE90W as
per my S2000 owners manual.
Maybe the person I talked to was wrong I don’t know???

I also stated that some not all brands fall below standards. I said it would
be best to check your brand of choice specs. Red Line has a high number it is
one of the better brands to use as is Amsoil.

I am not an oil guy I just read that the 75/110 is a better choice and covers
a better range.

I told my self never get into a oil thread on line you will always find strong
opinion that can escalate into hard debate or hard feelings.
I don’t want anyone to be angry with me over what I was told via a phone call
4+ years ago, I am thinking this information was wrong.
I do not know everything about oil. I only know what I was told and what I
read and use this information as what I think best for me. Seeing the numbers
I still feel that the 75/110 is the better choice.

customer
service at Honda motor Co USA some years ago and was given a range of 19 to
32 cst as a safe range for general use of the
S2000 Differential.

This
sounds like the old 140 weight spec...

Quote:

Originally Posted by S2KPuddyDad

I also stated that some not all brands fall below standards.
I said it would be best to check your brand of choice specs.

I
doubt that any name brand oil does NOT meet standards. Even if they did, I
doubt that they would pubicize specs that didn't
meet standards.

Quote:

Originally Posted by S2KPuddyDad

I am not an oil guy I just read that the 75/110 is a better
choice and covers a better range...
Seeing the numbers I still feel that the 75/110 is the better choice.

I'm not trying to bust your chops. I've seen your other posts,
and the only reason I would NOT send a differential to you would be if I
could find somebody out here on the Left Coast who could do it...

I'm just trying to make sure I'm using the right gear oil.

FWIW, I doubt that using 110 weight instead of 90
weight oil would make much difference (maybe a slight loss of power and
efficiency).

I have done a lot of reading as a result of this thread and
have come to the conclusion that 75-110 is a better choice than 75w-90 and
should be my first choice for my next differential change but this time I am
going with Castrol 75w90.

I should mention that my differential was recently replaced for a whine at 50
and 70mph.

I told my self never get into a oil thread on line you will
always find strong opinion that can escalate into hard debate or hard
feelings.
I don’t want anyone to be angry with me over what I was told via a phone
call 4+ years ago, I am thinking this information was wrong.
I do not know everything about oil. I only know what I was told and what I
read and use this information as what I think best for me. Seeing the
numbers I still feel that the 75/110 is the better choice.

No one is attacking you or angry with you. You were just a bit
adamant that 19.5 was the correct number and we had no idea where it had come
from.

But since that number came from a phone call to Honda, I'll just try emailing
Mazda with a very specific question and hopefully it'll get bumped to a
lubrication engineer who understands what I'm asking. If that doesn't work
I'll just drain the oil and have the viscosity tested.

As an aside to those that want to use 75W-110, but would prefer Mobil 1, mix
75W-90 and 75W-140 in equal parts and you should have a nice 75W-110.

Just
read the manual for myself. It specifies the following grades as acceptable
for the rear: API GL-5 of viscosity SAE 90, 80W-90, or 75W-90. Unless Mazda
gets back to me with a specific number, these are the specs I will follow.

And I just checked, and the Redline 75w90NS I put in my
transmission has a viscosity of 15.6cSt, and the Redline 75w90 for the
differentials is 16.4. I'll sleep well tonight!Bimmer

Bimmer,.. Thanks for the numbers on the Redline
75w-90. I have that in my diff now, so I feel better about that reading the
above.

But I wanted to comment on the 75w-90NS you have in the tranny. I first tried
that in my previous 02 Miata 5spd and was not happy with it. It was a little
better than the "dino" lube,
but still not good. I called and spoke with Dave at Redline, and he explained
"why".

Dave told me the 75w-90NS is a compromise lube. It has some anti slip agents in it to help the snychros
grip, but it also has some slip agents in it to help ring and pinion gears to
slip. This is a compromise lube for TRANSAXLES. He did not recommend it for a
separate manual transmission.

I then drained it out of my tranny and installed the Redline
"MT-90", 75w-90 fluid. After the lube washed the slip agents off of
the snychros, it made a big difference in the
smoothness of the shifting. I loved it!

So if you are using the 75w-90NS in a transaxle, then you are OK. But I was a
lot happier with the MT-90 in that 5 spd Manual
Transmission.

Isn't that is a was?
19.5 is conveniently close, but not too, to Amsoil's
20.3 for it's 75W-110

Yes, I should have said "was". The information
available is confusing, but what I've been able to find is that the old 90W
had a range of 13.5-24 (average 19.5). In 2006 sae
j306 changed to narrow the range of 90W to 13.5-18.5 and added 110W 18.5-24 cSt.

For example, under the old classifications, if the optimum
100°C viscosity for an axle was 19.5 cSt, the OEM
would normally specify an SAE 90 lubricant.

Sooooo, Amsoil's 75W-110
falls in the range of the "old" 90W and would appear to be a good
selection assuming that Mazda's recommendations were based on the old 90W
standard.

Given that I have an 06 which was actually built in 05 and probably designed
before that, what are the chances that my Owner's manual was referring to the
old SAE J306 standard or the revised 2006 version?

Given that I have an 06 which was actually built in 05 and
probably designed before that, what are the chances that my Owner's manual
was referring to the old SAE J306 standard or the revised 2006 version?

Since our rear requires a SAE 90W you need a cst of 19.5
This is why the change and why the old 75/90 is the new 75/110
You need to use 75/110

Also note your owners manual states no 75/90
available through Mazda for your differential
SAE 90W or 80/90W as a second
2 main reasons for diff failure is driver error and inadequate lubrication

while the 19.5cSt number you are getting is right
in the middle of the pre-revision 90wt range, it does not necessarily mean
that you need a 19.5cSt lubricant to adequately lubricate a miata diff.

the old range for 90wt was 13.5-23.9. since the miata diff specs were done during this period, it is safe
to assume that a 13.6cSt lubricant will do the job. most
of the XXw90 and straight 90wt lubricants are all formulated toward the light
end of the scale (one of the very reasons that the revision took place). since the current miata still
specs a 90wt, i am fairly certain that you don't
need a 19.5, or a 22cSt oil to adequately lubricate the rear end. as most people have been using a 13.5-15cSt lubricant all
along, and there have been no rash of diff failures, i
would be happy to call that empirical proof and move on.

a heavier lubricant in the rear end will just make
it run hotter, and use more fuel.

for those interested in the mechanics of it, a very
good table that i have posted before is

Since changing the differential oil was on my
to do list for today and I've been reading this thread, I stopped at
the dealer before picking up lubricant. The service manager (who I have a
fair amount of respect for), looked at a couple of factory books, looked
online (at what I don't know) and said that the recommendation is as printed
in the owners manual of my 08. He then pointed to a
55 gallon drum of Castrol 80/90 which is what they use for a rear
differential change, and then pointed to a 55 gallon drum of something else
from Mazda which is for front wheel drive systems only. That was good enough
for me, bought a quart of Castrol 80/90 from the dealership and will put that
in there and drive with peace of mind. Now, I have an automatic so no LSD so
others may benefit more form something else.

the
old range for 90wt was 13.5-23.9. since the miata diff specs were done during this period, it is
safe to assume that a 13.6cSt lubricant will do the job. most
of the XXw90 and straight 90wt lubricants are all formulated toward the
light end of the scale (one of the very reasons that the revision took
place). since the current miata
still specs a 90wt, i am fairly certain that you
don't need a 19.5, or a 22cSt oil to adequately lubricate the rear end. as most people have been using a 13.5-15cSt lubricant
all along, and there have been no rash of diff failures, i would be happy to call that empirical proof and move
on.

If the notion of trading a little gas mileage for more diff
protection bothers you, by all means buy the 90. If the notion of trading a
little diff protection for some gas mileage bothers you, buy the 110 (or even
140).

Like most things in life, it's a trade-off -- neither decision is right or
wrong.

If the notion of trading a little gas mileage for more diff
protection bothers you, by all means buy the 90. If the notion of trading a
little diff protection for some gas mileage bothers you, buy the 110 (or
even 140).

Like most things in life, it's a trade-off -- neither decision is right or
wrong.

jim (tired of a simple
decision becoming a running gunfight... )

The annoying thing about this argument is that neither side
really has any data. Neither side has any used oil analysis to prove that
either oil is better or worse. It really boils down to one question,
do you think you're more clever than the engineer who designed the car?

Not quite, Raven. The question is "Do you have the same
priorities as the manufacturer?" It's not that you or they "know
more". The manufacturer has a lot of criteria that you don't -- cost, emissions, mileage, corporate partnerships, etc.
It's legitimate to think of the manufacturers recommendations
as "minimums", and sometimes they're even better than minimum, but
their goal isn't to recommend "the best" for you -- that's
your job.

The great thing is that no one can make anyone else put anything in their
car.

Follow the owner's manual. Don't change it all. Drain it, run it through a
coffee filter and put it back. Whatever puts a shine on your armor. You can follow the advice of the butcher, the baker
or the candlestick maker.

Me personally? I'd follow the advice of the guy who builds rearends for a living.

I'm afraid that this wasn't for a Miata, it was for my '02
Subaru Impreza (I think I mentioned this above
somewhere).

FWIW, Redline's 75w90NS (and one quart of the Redline's Lightweight Shockproof "Smurf Blood")
is highly praised on the Subaru forums for the manual transaxles, and it made
shifting much easier in my car.

The annoying thing about this argument is that neither side
really has any data. Neither side has any used oil analysis to prove that
either oil is better or worse. It really boils down to one question, do you think you're more clever than the
engineer who designed the car?

i have
one, just haven't sent it in yet, 60k on my 02LS. hopefully
go out in the mail this week to blackstone.

Neither side has any used oil analysis to prove that either
oil is better or worse.

It wouldn't really prove anything if we did, either...

You'd need dozens is not hundreds of UOAs to come up with statistically
significant conclusions.

If you're trying to decide between 75w90 and 75w110, then the margins of
errors on UOAs are too big, and the other variables like manufacturing
tolerances in differentials, different brands of oil, and different driving
styles, are virtually insurmountable.

For example, Driver A's differential might show a lot more wear than Driver
B's, even if they use the same oil.

Or a sample of Brand A 75w90 after 30,000 miles might show a lot less wear
than a sample of Brand B 75w90.

Or a sample from a differential made on a Monday might show a lot more wear
elements than a differential made on a Tuesday, even if everything else
(weight and brand of oil, driving style) were the exactly the same.

Or you could send the same exact sample to several labs and have results that
vary 10-20%. Really.

So, pick whatever you guess will work best. Or do whatever your
brother-in-law recommends (or the opposite). Or whatever Mazda says. Or
whatever Puddy says. Or whatever Amsoil says. I
don't know...